This invention relates to a method of producing sodium perborate having a high active oxygen content and low bulk density.
Sodium perborate is frequently utilized in detergent products, many of which contain spray-dried components having mean particle sizes of about 30 to about 40 mesh and bulk densities of about 0.3 g./cc. These spray-dried components are produced by spraying a liquid solution or slurry through a heated zone and recovering discrete particles of detergent ingredients resulting therefrom. Since conventional perborate compounds normally have a higher bulk density of 0.75 to 0.95 g./cc. and extremely small particle size, they tend to segregate from detergent formulations made by mixing these sodium perborate materials with the spray-dried particles. Thus, there arose a need for sodium perborate having low bulk density.
The usual sodium perborate of commerce is a compound of the formula NaBO.sub.3.4H.sub.2 O which is also written as NaBO.sub.2.H.sub.2 O.sub.2.3H.sub.2 O. It is called "sodium perborate tetrahydrate" or more commonly "sodium perborate". This latter term will generally be used herein.
Methods of producing low bulk density perborate have been proposed. British Patent 1,065,832 discloses a process for making granulated sodium perborate having average particle sizes of 0.5 to 2 mm. and bulk densities of 0.3 to 0.5 kg./liter with active oxygen content of 11% or less. The material is made by spraying a sodium perborate, the water content of which has been reduced to less than 2.8 moles per formula unit, with an amount of water sufficient for the reforming of the sodium perborate tetrahydrate and containing a dissolved binder while the starting material is kept in moderate motion. Suitable binders are carboxymethyl celluloses, polyvinylpyrrolidone, carboxyl-group containing polyacryl- or poly-methacryl compounds, sugar, polyvinyl alcohols and dextrin.
U.S. Pat. No. 3,661,510 issued to Winkley on May 9, 1972 discloses sodium perborate crystals having a bulk density of from about 0.3 to about 0.6 g./cc. and a size larger than 60 mesh. The crystals are in the form of prismatic crystals having a specified length:width ratio. The sodium perborate disclosed therein is produced by reacting together, at 25.degree. to 45.degree.C., hydrogen peroxide and sodium metaborate in a molar ratio of H.sub.2 O.sub.2 :BO.sub.2 of 1:1 in the presence of externally prepared, preformed seed crystals of sodium perborate tetrahydrate having a length:width ratio of at least 3:1, and separating the resulting sodium perborate crystals from their mother liquor.
U.S. Pat. No. 3,403,984 issued to Mollard on Oct. 1, 1968 discloses a process for preparing a high strength hollow, unitary crystalline agglomerate sodium perborate having an apparent density between 0.35-0.50 by simultaneously introducing an aqueous sodium metaborate and a solution of hydrogen peroxide into a container and cooling the solution to below 5.degree.C. to produce fine crystals that agglomerate. These initial agglomerates thus formed are introduced into a continuously formed supersaturated solution of sodium perborate which maintains the agglomerates as a fluid bed in suspension. Thereby the initial sodium perborate agglomerates are covered with a layer of sodium perborate which increases the strength of the agglomerates.
Belgium Pat. No. 779,306 discloses the preparation of an abrasion-resistant sodium perborate monohydrate by moistening normal perborate monohydrate with at least 5 weight % water with respect to the perborate monohydrate at 20-90.degree.C. and then drying, first at 40.degree.C. and increasing the temperature to 90.degree.C. maximum. Moistening may be effected with pure water or with dilute aqueous solutions containing substances having no adverse effects on the active content or the speed of dissolution. Such substances include hydrogen peroxide. A rotating tube is disclosed as a suitable apparatus for accomplishing the required moistening.
Polish Pat. 40,766 issued to Stanislav Karczewski on Apr. 1, 1958 discloses adding up to 15% by weight hydrogen peroxide to a hydrated sodium silicate waterglass containing about 25% water. The hydrogen peroxide-silicate mixture is held for 12 hours after which the bulk very slowly swells and coagulates. The product obtained is dried at a temperature not exceeding 30.degree.C. for about 3 hours to yield a product having a light mass and emitting active hydrogen peroxide which during the preparation of the composition remains trapped in the mass.
In spite of the prior art methods, new or improved methods of producing sodium perborate having suitable properties for detergent formulation are desirable.